Fall armyworms have been showing up in rice in the past few weeks. Most of the calls about this regard rice that is starting to joint, but we also are catching some falls while sweeping for rice stink bug in heading rice.
Rice yields can be impacted pretty severely if large amounts of defoliation occur at this time.
We have conducted defoliation studies over the past three years looking at different levels of defoliation at multiple growth stages and planting dates. Based on this work, defoliation occurring at green ring can be a major concern. For April planted rice, we start seeing large amounts of yield losses occurring around 66% defoliation.
May- and June-planted rice are much more sensitive to defoliation at green ring. Both of these planting dates start significantly losing yield at 33% defoliation.
Major heading delays are associated with defoliation at green ring as well, ranging from 9 days to 30 days (see table). This can cause major headaches with harvest and overall management of the crop.
We do not suggest spraying armyworms if rice is heading unless severe defoliation or head clipping occurs.
Yield and days in delayed heading caused by defoliation at green ring.
|% Def.||Yield†||Heading Delay‡|
† Yield as percent of control (0% defoliation); ‡ Days.
If seed was over-treated with a diamide (Fortenza or Dermacor), the rice will most likely still be protected from armyworm feeding at green ring. If you do feel the need to make a foliar application for armyworms, Lambda-cyhalothrin (Warrior II, Lambda-Cy, Silencer, Kendo, etc.) works fine and will provide adequate control.
Odds are we are going to see more fields of late rice experiencing armyworm pressure this year. Late planted rice does not have the yield potential nor the time to recover from defoliation compared to early planted rice but can still tolerate up to 33% defoliation.
Be aware that if an application that is not warranted is made, it has the potential to make rice stink bugs worse by killing beneficial insects present in the field. Let’s be smart with our money, and only spray if the defoliation is getting severe.